Method and apparatus for controlling pulp digesters



' May 19, 1959 E BEN TT 2,887,427

METHOD AND APPARATUS FOR CONTROLLING PULFDIGESTERS Filed Aug. 16, 1954 2 Sheets-Sheet. 1

"'1 69 AZ? we 19/ o 91 I I o I NVENJFUR May 19, 1959 A. E. BENNETT 2,387,427

METHOD AND APPARATUS FOR CONTROLLING PULP DIGESTERS Filed Aug. 16, 1954 2 Sheets-Sheet 2 INVEN'FEIRILJ ANDREW 5. BEN/V 577" CM LLu-LM United States Patent-() METHOD AND APPARATUS FOR CONTROLLING PULP DIGESTERS Andrew E. Bennett, Weymouth, Mass, assignor, by mesue assignments, to Worthington Corporation, New York, N.Y., a corporation of Delaware Application August 16, 1954, Serial No. 449,930

8 Claims. ((31. 162-61) This invention relates to an improvement in the control of pulp digesters, particularly in respect to the relief of gases during the cooking process, and the method and means embodying this invention are especially suited for use in connection with digesters wherein the sulphate or soda process is utilized.

As is well known to those skilled in the art in the manufacture of paper pulp, a digester is filled with wood chips of a character suited to the product and is then charged with cooking liquor until the major portion of the chips are covered, the liquor being in the form of water to which carbonates, sulphides and hydroxides have been added to provide a caustic solution best adapted for the purpose. Heat is then applied to the mass, either directly by the introduction of steam atthe bottom of the digester with or without forced circulation, or indirectly by forcibly circulating the liquor from the digester to a heat exchanger and thence back to the digester, it being desirable with either method to raise the temperature rapidly to a predetermined maximum at which the cook temperature is preferably maintained until the pulp has attained the consistency desired.

It will be understood that whether a direct or indirect method of heating is employed, the efficient operation of a digester depends upon penetration of the cooking liquor into all of the chips and that this in turn depends on proper circulation throughout the entire mass. it has been found that during the heating up process of a. sulphate digester, quantities of air, together with volatile substances such as turpentine and the like, which vaporize as the cooking process proceeds, seriously impede circulation, and that these gases, although in gradually diminishing amounts, continue to accumulate for the balance of the cook. Owing to this accumulation of gases during the cook cycle, the actual temperature of the mass fails to adhere to the vapor pressure-temperature curve of the boiling solution, since the noncondensables and volatile fluids must be replaced by vapors from the boiling solution if the digester temperature is to conform with that of the vapor pressure-temperature curve. Thus the heat input is retarded whether the heat is provided directly by the introduction of steam or indirectly by forced circulation.

In my copending application filed on February 11, 1954, under Serial No. 409,699, I disclose an improved method and means for governing the admission of steam to the digester to provide a predetermined pressure-temperature cook schedule over a selected period of time by reference to the pressure-temperature conditions at the bottom of the digester. While the relief and/ or blowback mechanism hereinafter to be shown and described may be used with any form of heat input system, there are certain advantages to be obtained, as will be hereinafter set forth, by employing the apparatus herein disclosed in conjunction with the steam input system shown and described in the aforesaid application.

Heretofore it has been customary to relieve nonconrlensable gases from the digester by reference to the tem- And 2,887,427 Patented May 19, 1959 perature either in the upper section of the digester or in the relief line in fluid communication therewith and comparing said temperature with the pressure in the upper section of the digester. If the temperature falls below that required by the vapor pressure curve, the relief valve is opened to allow the escape of noncondensables and thereby maintain the ratio between the temperature and pressure within a predetermined value. If after the relief valve is opened wide and the predetermined ratio value is exceeded, it has been assumed that cooking liquor is being carried over through the relief line and that the screen, normally employed for the relief line to prevent the loss of wood chips and/ or pulp, has become clogged. It was then customary to introduce purging .fluid such as steam into the digester through the screen in order to clear it.

Referring to the relief of digester gases by reference to the upper section pressure and the upper section temperature, I have found from successive tests that the upper section temperature does not follow the vapor pressure curve in the upper section even though the screen is clear and an oversized relief valve is opened wide. This is particularly true in thefirst twenty to thirty minutes of the cook since that part of the cook period is the most critical due to sluggish circulation. I have found that under many conditions practically no upper section temperature rise is indicated for some time after a rise in pressure in the bottom of the digester occurs.

The patent to J. M. McAlear, No. 2,490,533, dated December 6, 1949, discloses a combined gas relief and iblowback control system which employs a relation controller responsive to relief line fluid temperature and to upper section digester pressure for operating a relief valve to maintain the ratio between the relief line temperature and the upper section pressure within a selected value, and if that value is exceeded, to blow intermittent blasts of steam through the screen into the digester until the screen is clear. While this system is a marked improvement on any system utilized prior to that time, I have found that a response to fluid temperature in the relief line combined with a response to fluid pressure in the upper section of the digester is not always a true indication of conditions which require the relief of gases or require the screen to be cleared. Tests show that while gas relief temperature adheres more closely to the upper section vapor pressure curve than the upper section temperature adheres to the said curve, the gas relief tem perature tends to vary somewhat from that curve even though the cock is performing satisfactorily. When this occurs, the relief valve is opened wider than required and blowbacks are initiated when the screen is not lo I have found from repeated tests that by utilizing a response to the temperature of fluids escaping from the digester combined with a response to pressure or temperature at the bottom of the digester for varying the opening of the relief valve as required to permit the escape of noncondensable gases, the temperature of the escaping fluids may be made to adhere closely to the bottom pressure vapor curve, allowing for the static pressure created by the weight of the digester contents, pro vided there is no carry-over of cooking liquor or clogging of the screen, and provided also that the steam input to the bottom of the digester does not exceed the con densing rate, whereby an excess bottom pressure is produced. I have also found that this is not true when reference is utilized to upper section temperature and to digester bottom pressure for relief purposes. Tests show that the upper section temperature may depart radically both from the bottom pressure vapor curve and the temperature of escaping fluids, especially in the early part of the cook, even though the steam input does not exceed the condensing rate and there is therefore no excess bottom pressure. For example, when the static pressure in the digester was compensated for, and the pressure at the bottom of the digester was p.s.i. gauge, a temperature bulb located adjacent the digester neck showed a temperature at that point of 212 degrees F., whereas a bulb located in the relief line showed a temperature of 239 degrees R, which temperature is the equivalent of a bottom pressure of 10 p.s.i. gauge. Thus the temperature at a point adjacent the digester neck was less than the temperature of the bottom pressure by 27 degrees F. Therefore if the temperature at that point is used as av reference, the relief valve would be opened wide and the vapors and liquor which should be re tained in the digester are lost. The term upper section as herein used refers to the upper portion of the digester body exclusive of the neck which is mounted on the top of the digester and serves as a passage for fluids ontheir way to the screen and relief line.

The term bottom section or bottom of the figure as herein used refers to the portion or space. of the digester body between the point at which the heat input mechanism is located and the discharge outlet for digested pulp. This space or area in the digester is critical for the purpose of the present invention.

The difference in temperature, noted above, of fluids in the upper section of the digester and of fluids escaping therefrom is occasioned by extremely sluggish circulation, a characteristic encountered in Wood pulp cooking, particularly in the early stages of the cook. Due to this sluggish circulation, vapors find their way through the cook mass along the line of least resistance, the location of the line of least resistance varying from time to time depending upon the particular condition of the cook mass. Therefore the vapors are channelled through a portion only of the upper section and a response to temperature at a point in the upper section other than that portion through which the vapors are passing, is not a true indication of circulation or of how the cook is progressing. Similarly a response to pressure in the upper section of the digester combined with a response to the temperature of the escaping fluids is not a true indication of cook conditions, since there is a definite lag between changes in pressure in the upper section and changes in pressure initiated in the bottom of the digester. By combining a response to changes in the temperature of escaping fluids with a response to changes in bottom pressure, a prompt and accurate guide to cooking conditions is obtained and this combined response may be utilized for relief purposes, and when required, for blowback.

I have found from tests carried on in connection with the commercial processing of wood pulp that the system herein disclosed increases the uniformity of pulp from a standpoint of permanganate numbers and physical tests. The reject rate is substantially reduced, and a material saving in steam is effected. Furthermore a larger volume of cooking liquor can be used which in itself improves circulation and results in a more easily washable pulp and in a reduction of salt cake loss per ton of pulp processed.

I wish it to be understood that While I preferably combine the means herein shown and described, or the equivalent thereof, for governing the relief of digester gases and for governing blowback, with means which govern the steam input to the digester by reference to bottom pressure, the relief and/ or blowback means may be combined with any form of steam input, since substantial improvement may be obtained thereby.

And I wish it to be also understood that while 1 herein show and describe a response to bottom pressure in connection with gas relief and also in connection with blowback, a response to bottom pressure is actually a response to bottom temperature since a change in bottom pressure reflects a change in bottom temperature corresponding to the well known vapor pressure-temperature curve for saturated steam.

It is an object of this invention to provide an improved means and method for governing the relief of gases from pulp digesters during the cooking process, and in connection therewith, when required, to provide an improved means and method for introducing purging fluid through the gas relief screen into the digester for the purpose of clearing same.

The invention will be more fully understood from the following description when taken in connection with the accompanying drawings in which:

Fig. 1 is a view, partly in diagram, of a pulp digester of the direct steam heated type showing automatic gas relief mechanism embodying my invention in combination with a preferred form of automatic means for governing steam input to the digester.

Fig. 2 is a front elevation, enlarged, of the controller, shown in Fig. 1, which governs the gas relief,

Fig. 3 is a front elevation, enlarged, of the controller, shown in Fig. 1, which governs the steam input, and

Fig. 4 is an enlarged fragmentary view showing the gas relief screen mounted in the neck of the digester.

Having reference to the drawings and particularly to Fig. 1 there is shown a digester 10 which includes a body portion 12, a cone-shaped bottom 12, a dome 13, and a neck 14 mounted on the dome having the usual detachable cover 15 to provide for charging the digester with wood chips. The bottom 12 has connected therewith a pipe 16, equipped with a hand valve 17 through which heated cooking liquor may be introduced and the digester filled to a level as indicated by dotted line 18. The pipe 16 and valve 17 are also utilized to discharge the finished pulp at the end of the cook.

The digester 10 is provided with a relief line 19 which is connected at 20 with the neck 14, the neck being herein shown as including a removable screen 21. The screen 21 is of cylindrical shape and at the bottom has an annular flange 22 secured thereto which is adapted to rest on an internal flange 23 suitably attached to the neck 14. The upper end of the screen 21 has an annular flange 24, also secured thereto, which engages the inner surface of a gasket 25 disposed between the cover 15 and the neck 14 to which the cover may be made fast, as by stud bolts 15 The relief line 19 has a relief valve 26 to permit the relief of gases from the digester and has connected therewith a blowback line 27 communicatring with a source of purging fluid such as steam, not shown, and having a valve 28 for governing the flow of steam through the screen into the upper part of the digester when required.

The digester 10 is also provided with a steam admission line 30 which connects a source of steam supply, not shown, With a perforated ring type manifold 31, or equivalent steam distribution device, in the upper portion of the bottom 12, by means of a plurality of pipes 32. Steam admission to the digester through the pipe 30 is varied by a valve 33 as governed by a time cycle pressure controller 34 in a manner now to be described.

Having reference to Fig. 3, the time cycle steam input controller 34 may be of any usual construction and, as herein illustrated, is similar to that disclosed in my copending application referred to above. The controller 34 has a backplate 35 and generally includes a Bourdon coil 36, a valve in the form of a flapper 37 and nozzle 38, a proportioning device generally indicated at 39 for modifying the effect of the initial response provided by the coil 36, and a clock-driven cam 40 which varies the setting of the controller. The coil 36 is fixed at its outer end to the backplate 35 by means of a block 41 and at its inner end is adapted to rotate a shaft 42, journalled at its inner end in a bottom member, not shown, secured to the backplate, and at its outer end in a member 43 also secured to the backplate. The fixed end of the coil 36 is connected by means of a pipe 44, a seal chamber 45. (see Fig.1), and a pipe. 46 to the cone bottom .12 of the digester so that the coil is subjected to the bottom pressure therein.

A water purging system may be employed to prevent the pipe 46 from becoming clogged with wood fibre. For this purpose the seal chamber 45 is connected to a pipe 47 having a suitable source of water supply under pressure, not shown. The pipe 47 may include a strainer 43, a reducing valve 49 downstream of the strainer, and a. restriction i downstream of the reducing valve, the

parts being herein diagrammatically illustrated. A check v valve 51 serves to prevent charge liquor, when under a higher pressure than that of the water supply, from being forced back through the supply line in the event of fail me of the reducing valve 49. A valve 52 in the pipe 46 is employed in connection with the water purging system which functions as will be hereinafter described.

Referring againto the controller 34, an arm 54 is secured to the shaft 42 so that it rotates therewith. The arm 54 is adapted to actuate an angle lever 55 about its fixed pivot 8 by means of a floating link 2. The angle lever 55 includes an indicator arm 56 which is adapted to cooperate with a suitable scale 57, and also includes an actuating arm 56 which positions the flapper 37 in respect to the nozzle 38 in a manner to be described. The floating link 2 consists of two members 3 and 5 which are adjustably connected as indicated at 4 to enable the length of the lever to be varied and therefore the controller to be zeroed in a manner to be described. And means for calibrating the movement of the angle lever 55 as required for any selected range of coil pressure change may consist of a series of openings 6 in the arm 54 and a series of openings 7, in the pointer 56, one end of the link 2 being pivoted at 9 in a selected opening in the arm 54, and the other end of the link being pivoted at 58 in a selected opening in the, indicator 56. In operation, an increase in bottom pressure in the digester causes the actuating arm 54 and pointer 56 to rotate counterclockwise, and on a decrease in the bottom pressure, the said members rotate in the opposite direction.

The proportioning device 39 may include a bellows 60 which is sealed at one end to a fixed mounting member 61 and at the other end to a disk 62. The propor tioning device also includes a helical coil spring 63 which is confined between a fixed mounting member 64 at one end. and a disk 65 at the other end, the disks 62 and 65 being connected by means of a rigid post 66. The interior of the bellows 60 communicates through a pipe 67 with a fluid operating pressure line 68 which connects with a source of regulated fluid pressure, not shown, by means of a ripe 6% having a restriction 7t). The line 68 also connects with the nozzle 36 which may be adjuste'd lengthwise of a mounting member 731 secured to the backplate 35, the line 68 having a flexible portion 72 to permit the nozzle to be moved to selected positions on the said member.

Fluid operating pressure in the line 68 is varied by means of the flapper 37 which has a pivotal connection 74 to the post 66 of the proportioning device at one end, and at the other end has a slot and pin pivotal connection 75 with one end of a floating lever 76. It will be understood that the capacity of the restriction 76 in the pipe 69 is less than that of the nozzle 38 so that the operating pressure in the line 68 is determined by the position of the flapper 37 in respect to the nozzle. The other end of the floating link 76 has a slot and pin pivotal connection 77 With the free end of the actuating arm 56 and midway its length has a pivotal mounting 78 on the upwardly extending arm 79 of a set point lever 80. One end of the set point lever 80 has a pivotal connection 81 with a bracket 82 secured to the backplate 35, and the other end of the lever provides an index pointer 33 which cooperates with a suitable control point setting scale 84. The lever 80 is rotatably positioned about its. pivot 81 by means of the cam 40 which coacts with a roller 35 mounted on a depending portion 86 of the lever, the parts being held in engagement by means of an extension spring 87 which is connected with the lever at one end and at the other end is connected to a pin 88 fixed to the backplate. The cam l-ll is mounted on a shaft 89 which is driven by any well-known type of clock mechanism, not shown. The cam may have a rotation period suited to the cooking conditions, a four hour rotation period for example, being common in sulphate digester cooking.

The steam admission valve 33 is actuated by a diaphragm 90 which partially defines an operating pressure chamber 91 having a connection with the operating pressure line 68 by means of a pipe 92. The diaphragm 90 is backed by a spring 93 which may be adjusted for any suitable operating range, such as from 3 p.s.i. to 15 p.s.i. As herein shown the valve is of the reverse acting type, being closed at 3 p.s.i. and fully open at 15 p.s.i. It will be understood that the output pressure range of the controller may be from 0 p.s.i. to 20 p.s.i. in accordance with the usual practice, and it will also be understood that while the simplified construction herein illustrated does not include a pilot commonly used to speed up the valve action, any Well-known type of pilot may be utilized for this purpose if desired.

In controlling a digester cook by the apparatushereinabove described, the cam is cut to conform to the cook schedule laid out by the mill personnel. It is customary in accordance with the practice at some mills to provide at the start of the cook a pressure rise of substantially 30 p.s.i. as rapidly as is possible without disturbing the cook, since the more rapidly the steam can be introduced, the better the circulation. From that point on the rate of pressure rise depends on the type of wood, the circulation of the liquor, and the end product obtained. The pressure-temperature rise period may vary from half an hour to three hours or longer and the holding period from half an hour to live hours depending on the mill practice and on the cooking conditions. i

in operation, if it be assumed that the static head pressure at the bottom of the digester, due to the weight of the chips and charge liquor, is 16 p.s.i., for example, the earn as is cut to provide an initial pressure which substantially corresponds to the static head pressure of 16 p.s.i. This pressure is indicated by the pointer 83 on; the scale 84. Since condensation accumulates in the digester during the cook period and an additional 2 p.s.i., for example, may be added so that the static pressure is increased to 18 p.s.i., if it be assumed that a maximum top pressure of p.s.i. is required during the holding period, the cam may be arranged to provide a bottom pressure of 118 p.s.i. during that portion of the cook. t

When the cook is started and the cam is set to provide a starting pressure of 16 p.s.i., the flapper 37 is positioned in respect to the nozzle 38 to provide a minimum output pressure in the diaphragm chamber 91 of the valve 33. The valve therefore remains closed. However, as the cam rotates countenclockwise, the pivotal connection 78 is raised, thereby rotating the floating link 76 in a counterclockwise direction around its pivotal connection 77. This movement of the link 76 rotates the flapper 37 clockwise about its pivotal connection 7 4 at the proportioning device, thus increasing the nozzle pressure. The increase in nozzle pressure is transmitted to the proportioning bellows 66 which expands and returns the flapper 37 to a throttling relation with the nozzle 38 at an increase in output pressure which increase is proportional to the increase in the pressure setting established by the cam 40. The diaphragm 9th of the steam valve 33 responds to this increase in output pressure and the valve is opened in proportional relation thereto to admit steam to the digester. As the pressure increases in the bottom of the digester, the Bourdon coil 36 rotates the actuating arm 54 counterclockwise, thereby swinging the link 76 clock wise and tending to move the flapper away from the nozzle to reduce the increase in output pressure initiated by the cam. When the lefthand pivotal connection 75 of the flapper moves down, the bellows 60 contracts, raises the righthand connection 74 of the flapper, and modulates the pressure change established by the coil 36 on a basis which varies with the width of the proportioning band. It will be understood that the width of the proportioning band may be varied by positioning the nozzle 38 along the support member 71, and that when the nozzle is positioned to the left, as viewed in Fig. 3, the proportioning band is narrowed and vice versa. Thus when the bottom pressure increases to the value established by the earn, the coil 36 functions together with the proportioning device 39 to maintain the pressure substantially at that value. If the rate of steam input exceeds the rate of condensation in the digester, and thereby tends to provide a pressure which is in excess of that established by the cam, the coil 36 turns the actuating arm 54 counterclockwise and reduces the nozzle pressure, whereby the steam input is decreased and the pressure returned substantially to the established setting. Thus a material excess in bottom pressure is prevented and the mass is not permitted to be lifted to a point where the liquor escapes through the relief line 19 and tends to clog the strainer 21 with chips or partly digested pulp.

It will be understood by those skilled in the art that when the controller 34 is located above the digester as herein diagrammatically illustrated, the purging system described above may be employed to fill the pipe 44 with water. This is accomplished by setting the reducing valve 49 at a pressure required to raise the water to the top of the pipe 44. At this time the valve 52 is closed. The connection between the pipe 44 and the coil 36 is broken and When the pipe is filled the end of the pipe is again secured to the coil so that a sealed joint is provided. When the digester has been charged and the liquor is at the level as indicated by the dotted line 13 for example, and the first cook is started, the valve 52 is opened. This permits a small flow of water, as determined by the restriction 50, to pass into the bottom of the digester, thereby clearing the pipe 46. When the pressure in the bottom of the digester reaches the pressure at which the reducing valve 49 is set, the reducing valve closes and the purging water is automatically shut otf. Since the pipe 47 is connected with the seal chamber 45 at a point above the connection of pipe 44, air which enters with the water through the pipe 47 is prevented from passing into the pipe 44 and is vented through the pipe 46 into the digester. The controller 34 may be calibrated to correct for the difference in liquid head in the digester and in the pipe 44 by means of the lengthwise adjustment provided by the floating link 2. Thus the controller may be zeroed at the liquor level indicated by the dotted line 13. Furthermore the controller 34 can be calibrated to compensate for the rise in liquid head due to steam condensation accumulating during the pressure rise period by suitably positioning the ends of the floating link 2 on the arm 54 and on the indicator 56. On this basis the cam 40 can be cut to operate from an initial bottom pressure of zero to the maximum holding pressure desired.

The gas-off control consists of a pressure-temperature relation controller 95 which may be of any usual construction and as herein shown is similar to the controller disclosed in the United States patent to McAlear referred to above. The controller 95 is adapted to actuate the relief valve 26 as required to maintain a set relation between the pressure in the bottom of the digester and the temperature of the gases escaping from the digester. The relief valve 26 includes an operating pressure chamber 96 which is partially defined by a diaphragm 97, the pressure in the chamber being opposed by a spring 98 in the usual manner. As herein illustrated, the parts are so arranged that the valve is normally closed, i.e.,

the valve is opened by an increase in operating pressure. Means for connecting the pressure chamber 96 of the valve with the output pressure of the controller is in the form of a line 99 which is governed by a valve 100 to be more fully described. The controller 95 is responsive to changes in gas relief temperature at a point downstream of the strainer 21 by means of a temperature sensitive bulb 101 which is connected to the controller by flexible tubing 102, the bulb being preferably mounted in a side connection 103 with which an elbow 104, forming part of the relief line, is provided. In accordance with my invention the controller 95 is also responsive to changes in the bottom pressure of the digester by means of a pipe 105 which communicates with the seal chamber 45 and thereby connects with the bottom 12 of the digester by means of the pipe 46 as described above.

Having reference to Fig. 2 of the drawings, the pressure-temperature relation controller 95, which is herein illustrated as the indicating type, will serve to describe the operating principle embodied in my invention. The controller mechanism is mounted on a suitable backplate and is provided with indicating pointers 111 and 112 and a setting indicator 113 which are adapted to cooperate with a scale 114. At 115 is an assembly support including a circular portion 116 and an extension 117, the circular portion being rotatably mounted on the backplate 110 by means of a fixed hearing ring 118 and having a gear segment 119 meshing with a pinion 120 which in turn is mounted on a shaft 121. The shaft 121 may be provided with a squared end suitable for a key, not shown, by which the assembly support arm may be rotated clockwise or in a counter direction. The setting indicator 113 is attached to the circular portion 116 and serves to indicate the ratio setting between the digester bottom pressure and the actual temperature, in terms of pressure, of the vapors being released through the relief line 19 in a manner to be described. The extension 117 may carry a nozzle 122 mounted on a support member 123 made fast to the extension. Cooperating with the nozzle 122 is a flapper 12.4 in the form of a flat spring secured at one end to a flapper arm 125, as by rivets 126. The flapper arm 125 is pivotally mounted on one end of the extension 117 and at the free end, as herein shown, is integral with a cam follower 127 which rests on a cam 128 to be more fully refer-red to. The nozzle 122 may extend into an opening 129 in the flapper arm for cooperation with the flapper 124 which is adapted to yield and prevent injury to the parts in the event that the normal operating range of movement is exceeded. While I have herein shown one type of nozzle-flapper arrangement, it will be understood that any usual construction may be employed. A convenient operating fluid, for example air, may be supplied under pressure to the nozzle 122 through the line 130 from a pipe 131, supplied from a source not shown, having a restricted orifice 132, the line 130 being connected with the valve 100. The nozzle-flapper arrangement provides a simple form of pressure system for operating the relief valve 26 either directly or indirectly through a pilot of any standard construction.

A ratio movement by which a selected ratio between the temperature, in terms of pressure, of fluids as they leave the digester and the fluid presssure in the bottom of the digester may be established and maintained, is comprised essentially of four members which are so arranged in respect to their relative positions and movements as substantially to constitute a parallelogram under all conditions of setting and control. These members comprise an arm rigidly secured to a pivot shaft 136 to which the pointer 112 is also rigidly secured; an am 137 rigidly connected to the pointer 111 which is in turn pivotally mounted on the shaft 136; and a lever 138 and link 139, the lever being pivoted at one end to the free end of the arm 135 and at its other end being arranged to carry 9 the cam "128, and thelink 139-being pivotally connected to the lever 138 and arm 137 as shown. The cam 128 may be adjustably secured to the lever 138 by means of a frictional mounting such as a screw 140 or the like.

The pointers 111 and 112 may be actuated by helical coils 144 and 145, respectively, secured to the plate 110 by supports 146, the former coil being responsive to digester bottom pressure and the latter coil being sensitive to changes in temperature of the fluids escaping from the digester. The coil 144 is responsive to digester bottom pressure by means of the pipe 1&5 as referred to above. The coil 145i is sensitive to changes in gas relief temperature by means of the tube 1112 and the bulb 101 which together with the coil may be filled with a suitable expansible fluid. The free end of the coil 144 is adapted to rotate an arm 147 which is pivoted on a shaft 148 at the center of the coil and is arranged to move the arm in a counterclockwise direction on an increase in pressure in the bottom of the digester. The pointer 111 may be operatively connected to the arm 147 by a link 149 by which means the pointer is positioned on the calibrated scale 114 in accordance with the pressure in the bottom of the digester as maintained by the steam input controller 34. The other pointer 112 may be in operative connection with the helical coil 145 by means of an arm and link 151 arranged in a similar manner to the connecting parts just described. The coil 145 positions the pointer 112 over the calibrated scale 114 and is adapted to indicate the relief gas temperature in terms of pressure. The cam 128 and ratio movement, including the pointer arms 135 and 137, link 139 and lever 13% cooperate to position the flapper 124 in relation to the nozzle 122 in accordance with the relative positions of the pointers 111 and 112. As shown in the drawings the cam surface is laid out substantially on the arc of a circle and the cam is positioned in such a way relative to the lever 138 and the follower 127 that a change in value of the bottom pressure is followed by the same change in value of the temperature in the relief line 19. This is accomplished by a change in output pressure in pipe 99 which results in a change in relief valve opening in a manner to be described. The cam action may be varied by swinging the cam about its mounting 141 or by changing the shape of the cam.

The adjustment and operation of the gas-off control is as follows:

The time cycle pressure controller 34 is set to maintain the desired time-pressure cook schedule. The gasoif controller 95 is adjusted to maintain a selected ratio between the temperature of the gases at the bulb 1131 and the temperature of saturated vapors of the liquid solution at the pressure in the bottom of the digester. This is accomplished byturning the pinion 120 to establish the initial relation between the nozzle 122 and flapper 124. Rotation of the assembly support 115 counterclockwise to move the setting indicator from zero on the scale 114 increases the ratio, and as the pointer is turned clockwise and approaches zero, the differential is decreased.

When the cook is started and steam is introduced into the bottom of the digester through the inlet valve 33 as governed by the time cycle controller 34, the pressure in the bottom of the digester is raised to a predetermined maximum over a selected period of time, is held at that maximum for the time interval required, and is then reduced at the end of the cook in accordance with the mill schedule. At the start of the cook, steam is introduced to the bottom of the digester at a relatively high rate, and since the mass is reasonably cool, the steam condenses rapidly. As the bottom of the mass becomes heated, circulation commences at the bottom of the digester and steam gradually permeates further into the mass, thereby resulting in further condensation. It is understood that closely packed wood chips tend to and open the valve.

restrict the natural circulation of themass, butsincethe controller 34 will not permit steam input to exceed the condensing rate, the prescribed increase in bottom pressure will be adhered to. Therefore a material increase in bottom pressure over the top pressure is prevented and a major factor in causing the cooking liquor to enter the relief line and clog the screen is avoided. Until the steam vapors penetrate through the mass, the difference in temperature of fluids in the relief line and the temperature corresponding to the bottom pressure is such that the helical coil 14% winds up turning pointer 112 and arm 1235 clockwise, thereby raising cam 128 and flapper 124. The nozzle pressure will thus be increased When the vapors have penetrated the mass and entered the relief line together with noncondensa'ole gases and the like, the temperature at the bulb rises, coil 14S unwinds turning pointer 112 counterclockwise to lower cam 12S and flapper 124 and reduce the opening of the relief valve. It will be understood that the relief valve is thereby governed to permit the escape of gases, such as air and non-aqueous vapors, as they collect at the top of the digester and enter the relief line and thereby maintains the temperature of relief line fluid within a selected ratio with the temperature of saturated vapor under the pressure obtaining in the bottom of the digester.

it will be understood that the controller functions to tend to maintain the gas relief temperature and bottom pressure within the selected ratio throughout the pressure rise and holding period. As the bottom pressure increases the pressure responsive coil 144 unwinds turning pointer 111 counterclockwise and with it arm 137, lever 138 and cam 128, thereby tending to raise the flapper to increase the operating pressure and open the relief valve. However, if the temperature at the bulb increases at the same ratio, the temperature responsive coil 145 will also unwind and turn the pointer 112 counterclockwise so that the arm lowers the calm: 128 and tends to reduce the nozzle pressure and close the relief valve. Under the conditions just referred to the parts are so arranged that the counterclockwise movement of the cam tending to raise the flapper 1'24 and open the relief valve is oifset by the downward movement of the cam tending to lower the flapper and-close the relief valve. Thus so long as the set ratio is maintained, the flapper-nozzle relation remains undisturbed and the opening of the relief valve is not varied.

It will also be understood that since the bottom pressure is maintained in accordance with a selected schedule and since the gas relief temperature adheres closely to that vapor pressure curve, a maximum circulation throughout the cook mass may be obtained which results in a more uniform pulp, less rejects, and a material saving of steam. Thus the method and apparatus for controlling steam input and gas relief in digester cooking is a marked improvement on any method and apparatus utilized heretofore so far as known to me. However 1 wish it to be understood that the method and apparatus herein disclosed for gas relief can be used to advantage with any form of heat input, since governing gas relief by reference to relief temperature and bottom pressure is a basic factor in the success of pulp cooking. Even though excessive bottom pressure occurs, thereby tending to upset the cook, the relief valve opening will be promptly varied with a view to returning the gas relief temperature and bottom pressure within the selected ratio, a result which cannot be so effectively obtained by other forms of gas relief, since the reference points are not a true indication of cooking conditions.

Referring again to the drawings, I herein illustrate a blowback system for use in connection with the gas relief in the event that adverse cooking conditions and carryover of liquor occurs and the screen becomes clogged. The steam valve 28 in the blowback line 27 is similar in construction to the relief valve 26 and like 11 that valve is adapted to open on an increase of pressure in its diaphragm chamber 160. The chamber 160 is in communication with a source of operating fluid under regulated pressure, not shown, by means of a pipe 161 in which a three-way valve 162 is mounted. The three way valves 100 and 162 are solenoid operated and may be similar in detail to those shown and described in the said United States patent to McAlear. When the solenoid 163 of the valve 100 is deenergized, the diaphragm chamber 96 of the relief valve 26 is connected with the nozzle pressure in the pressure-temperature relation controller 95, and when the solenoid is energized, the diaphragm chamber is cut off" from the nozzle pressure and connected with atmosphere. On the other hand, the valve 162 is arranged so that when its solenoid 164 is deenergized, the diaphragm chamber 160 of the blowback valve 28 is connected with atmosphere, and when the solenoid is energized, the diaphragm chamber is connected with the source of operating fluid pressure. Means for governing the operation of the solenoid valves is in the form of a pressure switch 165 which is governed by the nozzle pressure in the pipe 130 to which it is connected by means of a pipe 166. The switch 165 may include a bellows, not shown, which is opposed by a spring, also not shown, set to permit the bellows to expand when a predetermined pressure is exceeded and by means of a member 167, to close switch members 168 and 169 which connect a suitable source of electric current with the solenoids 163 and 164 all as shown.

The blowback mechanism functions to clear the screen 21 should occasion for doing so arise. It may be assumed that the pressure-temperature relation controller has an output pressure operating range from to 20 p.s.i., that the relief valve 26 is closed at a pressure of 3 p.s.i. and is fully open at a pressure of 14 p.s.i., and that the pressure switch 165 is set to close the switch contacts at 16 p.s.i. and to keep the contacts closed until the pressure again drops below that value. It may also be assumed that the blowback valve 28 is closed at a pressure of 3 p.s.i., is fully open at a pressure of 14 p.s.i., and that the fluid pressure in the pipe 161 is supplied at 20 p.s.i. Should the strainer 21 become clogged sufliciently to interfere with the flow of gases and render the relief valve ineffective to maintain the required pressure-temperature ratio, the temperature at bulb 101 will fall, thereby causing the controller 95 to increase the output pressure proportionally to the increase in the pressure-temperature ratio. When the pressure-temperature differential expands to a value at which an output pressure of 14 p.s.i. is reached, the relief valve 26 is opened wide. If the temperature at the bulb continues to fall, when the operating pressure arrives at 16 p.s.i., the pressure switch 165 will close the circuit and Will energize the solenoid 163 of the valve 100, thereby cutting off the diaphragm chamber 96 of the relief valve 26 from the nozzle pressure of the controller and venting the diaphragm chamber to atmosphere. At the same time the three-way valve 162 will close the vent to atmosphere of the diaphragm chamber 160 of the blowback valve 28 and will admit fluid operating pressure to the said diaphragm chamber. Thus the valve 28 will open wide to admit steam to the strainer 21, and the relief valve 26 will close to prevent the escape of steam through the relief line. The blast of steam will not only tend to clean the strainer but at the same time will raise the temperature at the bulb 101, and when the temperature at the bulb arrives at a value within the selected pressure temperature range at which the controller reduces the output pressure below 16 p.s.i., the switch will open the contacts 168 and 169, thereby closing the blowback valve 28 and opening the relief valve 26. If the strainer has been \freed by the blast of steam, the relief valve will function to maintain the set pressure-temperature relation. If however, the blast of steam fails to clear the strainer, the temperature at the bulb will again drop, and the cycle 12 just described will be repeated until the strainer is cleared and normal operation is resumed.

While I preferably utilize the blowback system in combination with the steam input system and gas relief system shown and described, the blowback system may be combined with the relief system described and any form of heat input, or may be usefully employed with any form of steam input and relief, since the reference to gas relief temperature and bottom pressure is a true indication of cooking conditions and avoids false blowback, i.e., blowbacks at times when there is no liquor carry-over and the screen is not clogged. And I wish it to be understood that the blowback valve may be governed by a temperature-pressure relation controller which is independent of the means utilized for relieving gases and is responsive to gas relief temperature and to digester bottom pressure, since any well-known form of snap-acting device which functions when the temperature-pressure ratio exceeds a selected value, may be employed to simul taneously close a relief valve and open a blowback valve and thereafter functions to open the relief valve and close the blowback valve when the ratio returns to the selected value.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

1. In control apparatus for a digester having a steam line connecting with the digester bottom and a relief line connecting with the digester top, in combination, a source of steam under pressure for the steam line, a steam valve in the steam line, a relief valve in the relief line, a controller, having a source of power, in operative connection with the steam valve, said controller being responsive to changes in pressure at the bottom of the digester and having time cycle means associated therewith for actuating the steam valve to raise the pressure at the bottom of the digester in accordance with a predetermined pressure rise schedule over a selected period of time, a strainer for the relief line upstream of the relief valve, a purging fluid valve communicating with a source of purging fluid under pressure and connected with the relief line downstream of the strainer and upstream of the relief valve for introducing purging fluid through the strainer into the digester, a first element responsive to fluid temperature changes between the strainer and the relief valve, a second element responsive to fluid pressure changes in the bottom of the digester, a ratio mechanism, having a source of power, in operative connection with the relief valve and governed by said elements to vary the opening of the relief valve to maintain the value of the ratio between the relief line temperature and the digester bottom pressure within a selected range of ratio value change over the said pressure rise period, and a third element governed by said first and second elements and in operative connection with the purging fluid valve and with the relief valve for closing the relief valve and opening the purging valve when the said ratio value exceeds the selected range of ratio value change during the said pressure rise period.

2. In control apparatus for a digester including a bottom portion, an upper section, and a neck portion mounted on the upper section having a relief line connected therewith, in combination, a source of steam under pressure having a connection with the bottom portion of the digester, a steam valve in said connection, means having a source of power in operative connection with said steam valve and responsive to pressure changes in the said bottom portion for increasing said pressure in accordance with a predetermined schedule over a selected period of pressure rise, a relief valve in the relief line, a first element responsive to changes in fluid temperature downstream of said upper section and upstream of said relief valve, a second element responsive to changes in fluid pressure in the said bottom portion, and ratio mechanism, having a source of power, in operative connection with said relief valve and governed by said elements for varying the opening of the relief valve in proportional relation to changes in the value of the ratio between said temperature and pressure over said period of pressure rise.

3. In control apparatus for a digester having a relief line in fluid communication with the digester upper section and having heat input means connected with the digester bottom portion for varying the pressure therein, in combination, a strainer for the relief line, a relief valve in the relief line downstream of the strainer, a source of purging fluid under pressure, a purging fluid valve communicating with said source and having a con-- nection downstream of said strainer and upstream of said relief valve for controlling the flow of purging fluid through said strainer into the digester, a first element responsive to changes in fluid temperature between the strainer and the relief valve, a second element responsive to changes in fluid pressure in the bottom portion of the digester, a ratio mechanism, having a source of power, in operative connection with the relief valve and governed by said elements for varying the opening of the relief valve in proportional relation to changes in the value of the ratio between said temperature and pressure over a selected range of ratio value change, and means in operative connection with each of said valves and governed by said first and second elements for closing the relief valve and opening the purging fluid valve when the said range of ratio value change is exceeded.

4. In control apparatus for a digester having heat input means which vary the pressure therein and including a bottom portion, an upper section, and a neck portion mounted on the upper section having a relief line connected therewith, in combination, a valve in the relief line, a first element responsive to changes in fluid temperature downstream of said upper section and upstream of said valve, a second element responsive to changes in fluid pressure in the said bottom portion, and ratio mechanism, having a source of power, in operative connection with said valve and governed by said elements for varying the opening of said valve in proportional relation to changes in the value of the ratio between said temperature and pressure.

5. In control apparatus for a digester having a relief line and having heat input means which vary the pressure in the digester, in combination, a strainer for the relief line, a valve in the relief line downstream of the strainer, a source of purging fluid under pressure, a purging fluid valve communicating with said source and having a connection downstream of said strainer and upstream of said relief line valve for controlling the flow of purging fluid through said strainer into the digester, a first element responsive to changes in fluid temperature between said strainer and said relief line valve, a second element responsive to changes in fluid pressure in the bottom portion of the digester, and a ratio mechanism, hav ing a source of power, in operative connection with the relief valve and governed by said elements for varying the opening of the relief valve in proportional relation to changes in the value of the ratio between said tem perature and pressure over a selected range of ratio value change, and means in operative connection with each of said valves and governed by said first and second element for closing the relief line valve and opening the purging fluid valve when the ratio between the said temperature and pressure exceeds a predetermined value.

6. The method of controlling the relief of gases from a direct steam heated digester for cooking wood pulp having a relief passage with a screen therein comprising the steps of, admitting steam to the lowermost portion of the digester over a selected period of time in accordance with a predetermined pressure temperature cook schedule, varying the admission of steam during the cooking schedule as the pressure temperature conditions vary at the lowermost portion of the digester to maintain the predetermined cook schedule, relieving gases from the top of the digester during said selected period of time, and regulating the rate of gas relief to maintain the temperature of the relief gases within a selected ratio to the temperature of the saturated vapor at the pressuretemperature conditions at the bottom of the digester.

7. The method of controlling the relief of gases from a direct steam heated digester for cooking. Wood pulp having a relief passage with a screen therein comprising the steps of, admitting steam to the lowermost portion of the digester over a selected period of time in accordance with a predetermined pressure temperature cook schedule, varying the admission of steam during the cooking schedule as the pressure temperature conditions vary at the lowermost portion of the digester to maintain the predetermined cook schedule, relieving gases from the top of the digester during said selected period of time, stopping the relief of gases and introducing a purging fluid to clean the screen and the relief passage when the ratio of the temperature of the relief gases to the temperature of the saturated vapor at the pressure-temperature conditions at the bottom of the digester exceeds a preselected ratio range, and terminating the introduction of purging fluid and recommencing the relief of gases when the temperature of the relief gases issuing from the di gester falls within the preselected ratio range.

8. The method of controlling the relief of gases from a direct steam heated digester for cooking wood pulp having a relief passage with a screen therein comprising the steps of, admitting steam to the lowermost portion of the digester over a selected period of time in accordance with a predetermined pressure temperature cook schedule, varying the admission of steam during the cooking schedule as the pressure temperature conditions vary at the lowermost portion of the digester to maintain the predetermined cook schedule, continuously relieving gases from the top of the digester during said selected period of time, regulating the rate of gas relief to maintain the temperature of the relief gases within a selected ratio to the temperature of the saturated vapor at the pressuretemperature conditions at the bottom of the digester, stopping the relief of gases and introducing a purging fluid to clean the screen and the relief passage when the ratio of the temperature of the relief gases to the temperature of the saturated vapor at the pressure-temperature conditions at the bottom of the digester exceeds the preselected ratio range for regulating the rate of gas relief, and terminating the introduction of purging fluid and recommencing the relief of gases when the temperature of the relief gases falls within the preselected ratio range once again.

References Cited in the file of this patent UNITED STATES PATENTS 1,633,553 Allen July 21, 1927 1,831,206 Swanson et a1. Nov. 10, 1931 1,911,145 DeMers May 23, 1933 1,918,181 Cram July 1, 1933 2,359,714 McKay Oct. 3, 1944 2,395,357 Trawick Feb. 19, 1946 2,402,705 Short June 25, 1946 2,451,073 Cowherd Oct. 12, 1948 2,490,533 McAlear Dec. 6, 1949 OTHER REFERENCES Ehrisman: Automatic Digester Control. Tappi Section, No. 16, 1944, pages to 198, Paper Trade Journal, volume 119, No. 20. 

6. THE METHOD OF CONTROLLING THE RELIEF OF GASES FROM A DIRECT STEAM HEATED DIGESTER FOR COOKING WOOD PULP HAVING A RELIEF PASSAGE WITH A SCREEN THEREIN COMPRISING THE STEPS OF, ADMITTING STEAM TO THE LOWERMOST PORTION OF THE DIGESTER OVER A SELECTED PERIOD OF TIME IN ACCORDANCE WITH A PREDETERMINED PRESSURE TEMPERATURE COOK SCHEDULE, VARYING THE ADMISSION OF STEAM DURING THE COOKING SCHEDULE AS THE PRESSURE TEMPERATURE CONDITIONS VARY AT THE LOWERMOST PORTION OF THE DIGESTER TO MAINTAIN THE PREDETERMINED COOK SCHEDULE, RELIEVING GASES FROM THE TOP OF THE DIGESTER, DURING SAID SELECTED PERIOD OF TIME, AND REGULATING THE RATE OF GAS RELIEF TO MAINTAIN THE TEMPERATURE OF THE RELIEF GASES WITHIN A SELECTED RATIO TO THE TEMPERATURE OF THE SATURATED VAPOR AT THE PRESSURETEMPERATURE CONDITIONS AT THE BOTTOM OF THE DIGESTER. 